Information provided by the American Academy of Ophthalmology
The Eye M.D. Association

REFRACTIVE SURGERY: A CLOSER LOOK

How the eye works

Light rays enter the eye through the clear cornea, pupil and lens. These light rays are focuses directly onto the retina, the light-sensitive tissue lining the back of the eye. The retina converts light rays into impulses, send through the optic nerve to your brain, where they are recognized as images. About 70 percent of the eye’s focus in power comes from the cornea and 30 percent from the lens.

Many forms of refractive surgery improve vision by permanently changing the shape of the cornea, the clear front window of the eye. Other procedures involve implanting a lens insider your eye.

In an eye with myopia (nearsightedness), the corneal focusing power is too strong for the eye’s overall length. Instead of focusing on the retina, images fall in front of it, and vision is blurry.

When treating myopia, certain refractive surgery techniques reduce the curvature of the cornea to lessen the eye’s focusing power. Images that are focused in front of the retina, due to an elongated eye or steep corneal curve, are pushed closer to or directly onto the retina following surgery.

In an eye with hyperopia (farsightedness), the corneal focusing power is too weak for the eye’s overall length. Instead of focusing on the retina, images focus beyond the retina, and vision is blurry.
When treating hyperopia, certain refractive surgery techniques make the cornea steeper to increase the eye’s focusing power. Images that are focused beyond the retina, due to a short eye or flat cornea, are pulled closer to or directly onto the retina following surgery.

Astigmatism occurs when the cornea is more curved in one direction than the other, like the shape of a football. If astigmatism is significant, light passing through the cornea is scattered. Images reaching the retina are distorted and vision is blurred. When treating astigmatism, refractive surgery techniques selectively reshape portions of the cornea to make it symmetrical and smooth, like the shape of a basketball, so that images focus clearly on the retina.